Process for the production of basic bessemer steel low in nitrogen



March 2, 1954 TOTAL LIME 177/0705 BESSEMER STEEL LOW IN NITROGEN Filed March 31, 1952 v INVENTOR. Fa def Grae/ 1 sin Patented Mar. 2, 1954 PROCESS FOR THE PRODUCTION OF BASIC BESSEMER STEEL LOW IN NITROGEN Rudolf F. Graef, Oberhausen, Germany, assignor to Hiittenwerk Oberhausen Aktiengesellschaft, Oberhausen, Germany Application March 31, 1952, Serial No. 279,602

7 Claims.

The present invention relates to steel making and, more particularly, concerns the production of steel low in nitrogen according to the basic Bessemer process.

It is known that converter steel produced by the usual basic Bessemer process cannot be used for many purposes, especially when the steel is to be subsequently subjected to considerable cold working. It is also known that this disadvantage, which has excluded the use of converter steel for the manufacture of many things, is the result of the relatively large nitrogen content of the steel. This nitrogen content in turn is the result of blowing air through the iron in the converter and, being about 0.010 to 0.025%, is higher than in other steels, such as open-hearth or crubath or by supplying the blast laterally instead of through the bottom of the converter. Also, attempts have been made to reduce the nitrogen absorption by altering the shape of the converter and the distribution of the blast in the bath or by enriching the blast in oxygen or gases which give off oxygen or by adding oxidic ores while simultaneously reducing the temperature during the blowing process. None of these attempts, however, have hitherto led to satisfactory results from both the technical and economic points of view.

From previous experience it has been determined that the nitrogen absorption in the usual Bessemer process is particularly large during the dephosphorisation period'which follows the decarburisation period, and increases'as the temperature of the contents of the converter rises during the dephosphorisation. In addition to the influence of temperature, however, an important part is played by the tendency of the iron to reach the saturation figure for nitrogen in iron as quickly as possible, this figure being substantially independent of temperature and being, for example, about 0.046% at 1,600 0. and about 0.048% at 1,700 C. Often the practice has consisted in a simple but inadequate effort to keep the temperature of the bath low by the addition of a cooling agent during the dephosphorisation and in this way to counteract excessive adsorption of nitrogen. This step alone, however, does not completely offset the tendency of the iron to take up nitrogen during the blowing and apart from the insufiiciently reduced increase in nitrogen it often brings with it too low a bath temperature and in consequence considerable difiiculties in casting. Moreover, a not unimportant part of the melt stays behind in the casting ladle. The good yield of steel accordingly drops and the smooth flow of the production is interrupted by troublesome additional work.

Lime is commonly added in lump form to a Bessemer converter before the pig iron is charged into the converter.

The present invention, which is concerned with the production of steel low in nitrogen by the basic Bessemer process, is based on a new discovery, namely that by adding powdered lime to the blast the tendency of the bath to take up nitrogen from the blast during the dephosphorisation period can be largely suppressed even at a comparatively high bath temperature. In the normal Bessemer process the phosphorus content of the iron bath is reduced by only an insignificant amount during the decarburisation period so that the removal of phosphorus really begins only when the decarburisation has ended. In fact lime charged into the converter in lumps can only gradually react with the oxidation products, so the slag takes up only a little phosphorus during the decarburisation period. Lime which has not reacted with oxidation products has little fluidity, but remains as solid pieces in the interior of the bath. Because of this lack of reactivity, phosphoric acid formed during the decarburisation period is not bound by the slag but is actually reduced in part by the carbon, so that phosphorus re-enters the bath. Nearly all the phosphorus must therefore be removed during the dephosphorisation period proper after the decarburisation.

In order to reduce waste from the converter and correspondingly to increase the yield, any supercooling of the bath must be avoided during the normal Bessemer process. During the normal Bessemer process, however, supercooling of the bath can usually be avoided only in the first and last minutes of the blowing process. Between the beginning and the end of the process,

however, there is'a sharp drop in the carbon con tent which reduces the fluidity of the bath and during which lime initially charged into the furnace in lumps has still not formed any freeflowing slag. As a result of adding lime in large pieces supercooling and Waste occur.

In the invention on the other hand, the lime is supplied in a much more reactive form, namely as a powder in the blast, and, moreover, in approximately such amounts throughout the blowing process as are necessary to bind those products of combustion which will react with lime and will maintain that free fluidity of the contents of the converter which is required for success of the whole process. At any time the amount of lime introduced into the blast is preferably not more than 2.5 kg. per cubic metre of blast, but the actual amount varies. During the initial stage in which silicon is burnt the supply of lime should be substantial so as to combine with all the silica and form a calcium silicate slag. During the next period, in which normally decarburisation occurs practically exclusively, the supply of powdered lime is continued at a reduced rate (with or without total interruption) so that there is reactive lime to bind some of the phosphoric acid. By not adding too much lime at this stage the slag remains fluid. In consequence of this addition of a limited amount of powdered lime, considerable dephosphorisation of the iron occurs during the decarburisation. It is found that as a result the decarburisation is delayed, a fact which is obviously due to part of the oxygen of the blast being required for the oxidation of the phosphorus so that there is only a lesser amount of oxygen available to oxidise the carbon and in consequence the decarburisation is prolonged. This results in the period in which carbon monoxide is formed as a result of the combustion of carbon being prolonged and extending substantially into the period of dephosphorisation proper. However, so long as carbon monoxide is produced the absorption of nitrogen by the bath is suppressed, since the partial pressure of nitrogen in the rising current of gas is small so long as the carbon monoxide is formed at every level of the bath, which or course is in a state of agitation. The additional mechanical agitation of the bath produced by the carbon monoxide actually tends to drive out nitrogen already present in solution in the iron.

By prolonging the decarburisation period that part of the dephosphorisation period in which no production of carbon monoxide takes place is thus automatically shortened. By shortening the period in which there is merely dephosphorisation and no decarburisation that part of the ,1

blowing period during which nitrogen can be taken up by the bath is reduced by an equivalent extent.

By regulating the addition of the line to the blast the whole process is controlled in the way which best corresponds to the physical and chemical requirements in the converter for the process of producing steel low in nitrogen.

It is necessary also to regulate the temperature of the bath, and this is done by blowing iron oxide or other powdered cooling agent into the bath or introducing scrap or other cooling agent through the mouth of the converter so as to prevent overheating. In particular, after the first period of blowing in lime, the heat produced must not lead to a disadvantageous increase in the temperature of the bath; therefore, it is de sirable to add a cooling agent during this period. For example, iron oxide or iron ore in finely divide-d form added to the blast, or ore or scrap introduced from above through the mouth, may be used at this stage. The preferred process is therefore characterised by a sequence of steps, correlated with the composition and temperature of the bath and the time, comprising adding lime during the initial blowing period, then adding a cooling agent while reducing or omitting the addition of lime, and finally adding lime again during the dephosphorisation. If during the latter part of the blowing period there is again an increase in the bath temperature, which would be unsatisfactory so far as the nitrogen absorption is concerned, it is desirable again to add a cooling agent (ore or scrap), either alone or together with lime, through the mouth of the converter or to the blast. In the control of the course of the reaction in the converter by dosing into the blast solid materials which serve to slag incidental elements or to cool the iron bath or both, materials which give on gases, e. g. limestone or iron carbonate, may also be used. Solid materials which give off gases after being introduced into the converter reduce the partial pressure of nitrogen in the blast and produce the same effect as prolongation of the decarburisation period and help to keep the nitrogen content of the bath low.

Steels produced by the process described above have nitrogen contents substantially below those of normal converter steels. Reduction in the nitrogen content can be additionally ensured if the blast is enriched in oxygen, especially during the dephosphorisation period, and if the oxygen supply to the iron bath is increased, advantageously at least to correspond to the oxygen consumed in burning the incidental elements accompanying the iron, particularly silicon, manganese, carbon and phosphorus. The oxygen addition aims at producing an extreme lack of equilibrium in the boundary layer between on the one hand the bubbles of blast rising up through the iron bath and on the other hand the liquid metal on the other side of the boundary layer. The solvent capacity of the boundary layer is claimed by the iron oxide, which is available in excess. In consequence, and by reason of the incidental elements still dissolved in the iron, the passage of the substantially less readily soluble nitrogen through the boundary layer into the metal bath is restricted. When the usual blast is employed, the desired disequilibrium of the three neighbouring phases is less easily maintained because of the-smaller supply of oxygen so that at least in the upper part of the bath free nitrogen can be absorbed from the bubbles of blast not only in the boundary layer itself but also upon passage" ing which it is merely dephosphorisation that:

takes place is reduced by means of the invention,

it is just in this period that the addition of,

oxygen to the blast is particularly advantageous.

It is further found that by the method described it is possible to produce converter steels which are characterized not only by a low nitro gen content, e. g. 0.010 or even as low as or less than 0.097%, but also by low phosphorus and. sulphur contents and can therefore be regarded as converter steels of high purity. The amounts of these impurities are particularly low when controlled additions of lime and cooling agent are made to the blast and are combined with enrichment of the blast in oxygen in the way described.

As one example, a converter charge consisted of 20 tons of pig iron of the following composition:

C Si M11 P S Percent 3.61 0.35 1.41 1.89 0.052

Lime dust was blown into the bath with the blast in the amounts shown in the accompanying drawing, which is a graph in which the abscissae are the minutes during which blowing has taken place and the ordinates are the total amount of powdered lime added to the blast. The figure shows three curves, of which that marked A represents the actual additions of lime, that marked B is the quantity which would be theoretically required to bind all the oxidation products capable of reacting with the lime and that marked C shows the lime which would be added before the charging of the pig iron in a normal Bessemer process. It will be observed that the total amount of lime added according to the invention was the same as the usual amount.- It will be seen also that lime was added for about the first minute and then discontinued for the second minute and thereafter added again until the end of the fourth minute. During this period the rate of addition was high as the lime was required to react with the silica. During the second discontinuance a cooling medium was added in the form of 4.00 kilograms of ore in the form of lumps. Immediately after this, that is to say, after minutes, the supply of powdered lime was begun again but at a slower rate, since at this stage the bath was essentially being decarbonised, and this lime supply was again cut off after 8 minutes. One minute later another 400 kilograms of ore was charged through the mouth of the converter as a cooling agent and after 10 minutes lime was again supplied at substantially the same rate as during the burning of silicon. This supply of lime was cut off again for 1 minute after 11 minutes and then continued at a slightly higher rate until 13 minutes from the beginning of the blowing. Thereafter it ceased and the blowing stopped at 14.7 minutes.

Specimens taken from a charge so treated showed the following carbon and phosphorus contents:

Carbon, Phosphorus,

Percent Percent Percent 0.04 0.32 0.638 0.025

0 Si Mn P s Percent 3.64 0.34 1.37 1.88 0.044

were charged into the converter. About 600 kilograms of lime were blown into the bath during the first 1.2 minutes and the same amount during the period from 2.9 to 4.3 minutes and again the same amount during the period from 5.7 to 6.9 minutes. Finally, during the period from 8.7 to 11.3 minutes about 1200 kilograms of lime were blown into the melt. In this case the cooling was effected by a single addition of 14 kilograms of ore in lump form at 8.3 minutes from the start of the blowing. The blast was enriched in oxygen in increasing amounts so that during the period from 3 to 9 minutes the average enrichment was 26% and from then to the end of the blowing the average enrichment in oxygen was 33%. The extent to which the slagging of the phosphorus delayed the combustion of carbon is clearly shown by the following table of carbon and phosphorus contents found in specimens taken at different times:

Carbon, Phosphorus, Percent Percent After 2.2 Min 3. 32 1. 86 After 5.3 IMin 2.05 1. 50 0. 26 0. 818 0.02 0. 480 0. 02 0. After 12.2 Min 0. 02 0.088

In this case the final steel had the following composition:

0 Mn P S N2 Percent 0.05 0.29 0.043 0.029 0.006

The process according to the invention can be varied by additionally blowing sand into the converter or adding it through the mouth of the converter as part of the cooling agent. This makes it possible, for instance, to increase the solubility of the slag in citric acid and therefore the value of the slag as a fertiliser. Instead of sand, acidic iron ore can be introduced into the converter as part of the cooling agent and also because of its silica content will increase the fluidity of the slag and the solubility of the slag in citric acid. The process therefore affords the possibility of using acidic ores, which previously have been regarded as undesirable and of verylittle use.

The process according to the invention is of importance not only in that it allows converter steel of low nitrogen content to be made but also because the weight of the charge can be increased and the waste reduced.

What I claim is:

1. A process of producing basic Bessemer steel low in nitrogen, which includes the steps of pouring molten pig iron into a basic Bessemer converter; blowing powdered lime into the converter with the blast in the first stage of the blowing process in approximately the amount required to bind the combustion products of silicon; carrying out a decarburization step and during the decarburization period continuing blowing powdered lime into the bath at such reduced rate as is required to bind the arising combustion products of a substantial portion of the phosphorus contained in the pig iron; completing the dephosphorization of the iron by again increasing the supply of powdered lime to the blast to approximately the amount required to bind the further arising combustion products of the phosphorus; and controlling the temperature of the bath by introducing a solid cooling agent into the bath.

2. A process according to claim 1, in which as cooling agent a solid powdered cooling agent is used.

3. A process according to claim 1, in which a solid powdered iron oxide is used as solid cooling agent.

4. A process according to claim 1, in which scrap is used as solid cooling agent.

5. A process of producing basic Bessemer steel low in nitrogen, which includes the steps of: pouring molten pig iron into a basic Bessemer converter; blowing powdered lime into the converter with the blast in the first stage of the blowing process in approximately the amount required to bind the combustion products of silicon; carrying out a decarburization step and during the decarburization period continuing blowing pow dered lime into the bath at such reduced rate as is required to bind the arising combustion products of a substantial portion of the phosphorus contained in the pig iron; completing the dephosphorization of the iron by again increasing the supply of powdered lime to the blast to approximately the amount required to bind the further arising combustion products of the phosphorus; and controlling the temperature of the bath by introducing a gas developing material into the bath.

6. A process according to claim 1, in which at least during the completion of the dephosphorization a blast enriched in oxygen is used.

7. A process according to claim 1, in which a solid cooling agent is used that contains silica.

RUDOLF F. GRAEF'.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,709,389 Davis Apr. 16, 1929- 1,826,497 Bicheroux Oct. 6, 1931 2,323,695 Webster July 6, 1943 2,502,259 Hulme Mar. 28, 1959 FOREIGN PATENTS Number Country Date 1,577 Great Britain of 1879' 3,010 Great Britain of 1879 3,714 Great Britain of 1879 25,251 Great Britain of 1912 

1. A PROCESS OF PRODUCING BASIC BESSEMER STEEL LOW IN NITROGEN, WHICH INCLUDES THE STEPS OF: POURING MOLTEN PIG IRON INTO A BASIC BESSEMER CONVERTER; BLOWING POWDERED LIME INTO THE CONVERTER WITH THE BLAST IN THE FIRST STAGE OF THE BLOWING PROCESS IN APPROXIMATELY THE AMOUNT REQUIRED TO BIND THE COMBUSTION PRODUCTS OF SILICON; CARRYING OUT A DECARBURIZATION STEP AND DURING THE DECARBURIZATION PERIOD CONTINUING BLOWING POWDERED LIME INTO THE BATH AT SUCH REDUCED RATE AS IS REQUIRED TO BIND THE ARISING COMBUSTION PRODUCTS OF A SUBSTANTIAL PORTION OF THE PHOSPHORUS CONTAINED IN THE PIG IRON; COMPLETING THE DEPHOSPHORIZATION OF THE IRON BY AGAIN INCREASING THE SUPPLY OF POWDERED LIME TO THE BLAST TO APPROXIMATELY THE AMOUNT REQUIRED TO BIND THE FURTHER ARISING COMBUSTION PRODUCTS OF THE PHOSPHOROUS; AND CONTROLLING THE TEMPERATURE OF THE BATH BY INTRODUCING A SOLID COOLING AGENT INTO THE BATH. 